GraphQL validation

Type system & Execution process allow server & client to effectively inform developers when an invalid query has been created.

• Custom Type (other than scalar, enum, etc) does contain field
• No fragment cycle (refer to itself)

GraphQL execution

Root Type represents all possible entry points into the GraphQL API.

The resolver process can return (and infer) directly trivial result. Ohterwise, in case of asynchronous resolvers we have to use Promise.
Each resolve function have got current object, potentially arguments and context in parameters.
Some resolve function can be executed concurrently by GraphQL.